In the field of steering shafts for motor vehicle steering systems, it is known to determine a torque which is input via an input shaft relative to an output shaft. The torque which is determined in this way can represent the basis for the determination of an assistance torque which is to be introduced into the steering system or an assistance force of a power assistance means for steering assistance of the driver. To this end, it is known to divide the steering shaft of the motor vehicle into an input shaft and an output shaft, the input shaft usually being connected to the steering wheel of the motor vehicle, via which a driver inputs the corresponding steering torque or the corresponding steering command into the input shaft. The output shaft is usually connected directly or indirectly to the steering pinion which, via a corresponding rack and track rods, transmits the steering movement to the steered wheels of the motor vehicle.
Power assistance means, for example electric power assistance means or hydraulic power assistance means, are usually attached to the output shaft, to the steering pinion or to the rack in order to input the corresponding assistance torques. Here, the respective power assistance means is actuated via the determination of the torque with regard to the output shaft, which torque is input into the input shaft by the driver via the steering wheel.
It is known here to connect the input shaft and the output shaft of a steering shaft by means of a torsion bar which is also called a torsion rod, and to determine the input torque from the relative rotary angle between the input shaft and the output shaft via a torque sensor. In the case of a hydraulic power steering system, this can take place, for example, via a rotary slide valve and, in the case of an electromechanical power steering system, this can take place, for example, via corresponding magnetic sensors.
Steering shafts for electromechanical power assistance means are used to measure the input torque. During the input of a steering movement via a steering wheel of the motor vehicle by way of the respective driver, a torque is input into the steering shaft, and, in particular, into an input shaft, in order finally to swivel the wheels of the motor vehicle. Here, the torsion bar which is arranged between the input shaft and the output shaft serves to transmit the input torque from the input shaft to the output shaft, said torsion bar being twisted by a defined amount which corresponds to the torque which is input via the input shaft, with the result that relative twisting between the input shaft and the output shaft is achieved. The power assistance means can be actuated accordingly from said relative twisting between the input shaft and the output shaft, which relative twisting can be sensed by way of the sensor.
Since the torsion bar is fixed both in the input shaft and in the output shaft, relative twisting accordingly takes place between the input shaft and the torsion bar and between the output shaft and the torsion bar. The twisting between the input shaft and the torsion bar is based on the fact that the torsion bar is held in a rotationally fixed manner in a bore at one end of the bore of the input shaft, and then extends through the bore toward the output shaft. Relative twisting between the torsion bar and the wall of the bore of the input shaft which lies opposite it accordingly takes place at the transition to the output shaft.
In order to prevent overloading of the torsion bar, the input shaft and the output shaft can be connected or coupled to one another via a loose positively locking connection, in such a way that direct positively locking engagement of the input shaft with the output shaft occurs if a maximum value for the elastic twisting of the torsion bar is exceeded.
In order to prevent oblique positions between the input shaft and the torsion bar, a bearing element is provided in the bore, by means of which bearing element the torsion bar is mounted with respect to the first shaft part. The torsion bar is usually mounted via a needle bearing or a sliding bush as bearing system in the bore of the output shaft. In order to prevent a falsification of the torque measurement here and at the same time to minimize noise formation which can occur, for example, by way of an existing bearing play, high tolerance requirements exist for said bearing elements.
DE 100 10 837 A1 has disclosed a rotary slide with a torsion bar fastening means without machining with the removal of material, in which a torsion slide which has a central axial bore with a wall and a torsion bar which is arranged coaxially in the bore are provided, and a needle bearing being provided between the torsion bar and the wall of the coaxial bore.